Apps from downloaded apk: why do some not work?

Blackberry 10.3.2.2836.
(Though Blackberry Virtual Expert says 2876. Why this annoying mismatch?!)
That's not my question. This is:
I do not like BlackBerry World (too Games oriented) and absolutely loathe Google Play.
I get my .apk files fom apk4fun or apkpure or apk-dl or ... excellent resources.
Most install and work superbly. Good examples are Netflix, Kindle, Audible, Firefox, Chrome, Wikipedia, Skype, WhatsApp, Glympse, Google Maps, XE Currency, WebEx, RingGo, MathsApp and half a dozen others.
Infuriatingly, whilst the .apk files appear to install (well, no error message) others do not: including BlinkBox, Amazon Video, TrueCrypt and All 4. Either they grey the screen forever, or repeatedly offer the Wait/Close option, or just nothing happens.
Why is this? I know 10.3 is "almost" Android without actually "being" Android. Is that all it is? It's stunningly head-bangingly tongue-strippingly annoying that a phone as versatile (and, plainly, as expensive) a BlackBerry Passport won't just "do everything", rendering it unarguably the best smartphone on the planet ... but it doesn't. (
Fergus

By using Google Play Store, you will get the precise version of the app that works on our version of Android. Many apks have multiple variants - and it's harder to get the right one on the other stores. Plus, you should read this:

But as others have stated, 66% of Android users have moved to newer versions than the one we have on BB10 devices - so our access to apps has begun to decrease rapidly. Plus, more and more apps require Google Play Services, which is officially forbidden on BB10. Workarounds exist to varying degrees of success.